1. Field of the Invention
The present invention relates to a gallium nitride (GaN) compound semiconductor and a method for manufacturing the same and, more particularly, a p-type GaN compound semiconductor, a method for manufacturing the same, and a light emitting device having the p-type GaN compound semiconductor.
2. Description of the Related Art
In recent years, a GaN compound semiconductor represented by a general expression A1(x)In(y)Ga(1-x-y)N (1.gtoreq.x.gtoreq.0, 1.gtoreq.y.gtoreq.0) has been watched with interest as an excellent material for a blue light-emitting device.
A blue light-emitting diode using the GaN compound semiconductors is made up of a plurality of p-type, n-type or i-type GaN compound semiconductor layers which are formed by the MOCVD (Metal Organic Chemical Vapor Deposition) method or the MBE (Molecular Beam Epitaxy) method on a sapphire substrate.
For example, a PN junction type blue light-emitting diode consists of a buffer layer which is made of a GaN layer formed on the sapphire substrate and a laminated structure which is made up of an n-type GaN layer and a p-type GaN layer both being formed on the buffer layer. In addition, a double-hetero (DH) structure blue light-emitting diode is composed of a buffer layer formed on the sapphire substrate, an n-type GaN layer formed on the buffer layer, a substantially intrinsic i-type A1(x)Ga(1-x)N (1.gtoreq.x.gtoreq.0) layer formed on the n-type GaN layer and called an active layer, and a p-type GaN layer formed on the i-type A1(x)Ga(1-x)N layer.
In this fashion, in order to manufacture an electronic device such as a light emitting device by use of the GaN compound semiconductor, there is necessity of forming at least a laminated structure which consists of n-type and p-type conductivity GaN compound semiconductor layers.
Normally, if undoped GaN compound semiconductors are to be formed by the MOCVD method, trimethylgallium (TMG), trimethylaluminum (TMA), ammonia (NH3), etc. are employed as a reaction gas, and hydrogen (H2) gas, etc. are employed as a carrier gas. At this time, a substrate temperature is set in the range of 800.degree. C. to 1200.degree. C.
If a monosilane (SiH4) gas, for example, is added as an n-type doping gas together with the reaction gas, a low-resistivity n-type GaN compound semiconductor can be formed.
However, even if a p-type doping gas, e.g., cyclopentadienyl magnesium (Cp2Mg) together with the reaction gas is added in the same manner as the case where the n-type GaN compound semiconductor is formed, a film obtained immediately after deposition (i.e., as-deposited film) is not a low-resistivity p-type GaN compound semiconductor. This is because Mg serving as the impurity is added to the film but it cannot be activated, so that such Mg cannot serve as an acceptor. As a result, such as-deposited film cannot be employed for the p-type GaN compound semiconductor layer constituting a light emitting device. Similarly, such tendency also appears even if other p-type impurity such as Zn is added.
The reason has not been sufficiently apparent why the GaN compound semiconductor, to which the p-type impurity is added, is difficult to exhibit the p-type electronic property. However, in general it has been understood that added Mg, Zn, etc. are bonded with hydrogen to thus generate Mg-H bond, Zn-H bond, etc.
More particularly, it has been supposed that NH3 employed as a nitrogen (N) source is dissociated to generate atomic hydrogen during when the GaN compound semiconductor is formed with the use of the MOCVD method, and then such atomic hydrogen reacts with Mg, Zn, etc. added as the p-type impurity to thus generate Mg-H bond, Zn-H bond, etc.
In Japanese Patent Publication No. 218625 of 1991, a method of activating the p-type impurity by forming the GaN film to which Mg as P-type impurity is added, and then annealing the GaN film at a temperature of more than 400.degree. C. has been disclosed. In this Publication (KOKAI) Hei 3-218625, such fact has been set forth that, according to this method, a carrier concentration serving as the acceptor (referred to as "acceptor concentration" hereinafter) of 8.times.10.sup.10 cm.sup.-3 in the as-deposited film can be improved up to the acceptor concentration of 2.times.10.sup.17 cm.sup.-3 by the annealing. It has been known that, because the Mg-H bond is dissociated by applying heat to the as-deposited film, the p-type impurity is electrically activated in this method.